Incremental or absolute position measuring systems are used for detecting the definite position of moved objects on machines, such as machine tools or wafer steppers, for example. In connection with this, the position measuring system must measure the position of the object at fixed definite times and inform the electronic control device which controls the movement sequence. The times are mostly defined by the electronic control device with the aid of trigger pulses. These trigger pulses are provided to the position measuring system, or to its electronic evaluation device, which then stores an internal count and triggers the A/D converters for signal interpolation by storing, or taking over, instantaneous values of the scanning signals with the same period, which are phase-shifted in respect to each other, and which are analog-digitally converted. At the end, the internal signal processing device in the electronic evaluation device outputs a measured position value which was not present exactly at the time of the triggering, but instead at a time which was displaced by the amount of storage time. Typical storage times are a few μs.
The continuously increasing displacement speeds, and the increased demands made on accuracy at the same time require, particularly at high speeds, increasingly shorter storing times, and above all an extremely small fluctuation of the storing time (storing jitter). The latter will be explained by the example of a wafer stepper, which was taken from an article by P. Kwan, U. Mickan, M. Hercher “Nanometergenaue Positionsmessung in allen Freiheitsgraden” (Position Measuring Accurate Down to a Nanometer in all Degrees of Freedom), F&M 108 (2000) 9, pp. 60 to 64. At a displacement speed of 2 m/s and a storing jitter of only 1 ns, the position uncertainty caused by this is already 2 nm, which represents a considerable loss of accuracy in connection with such applications. On the other hand, a storing jitter of less than 1 ns makes extremely high demands on the electronic evaluation device and the position measuring system. The following effects must be taken into account in connection with position measuring systems:                i) All analog amplifiers required for signal processing upstream of the A/D converter have limited bandwidths, and therefore delay the scanning signals to a considerable extent. Small amounts of drift of the components used because of the effects of temperature or aging affect the signal running times and therefore greatly contribute to storing jitter. Moreover, the signal running times are a function of the input frequency, and therefore the displacement speed, which can produce additional contributions to the storing jitter;        ii) The A/D converters also contribute to the storing jitter, because they do not measure the applied voltages exactly in relation to the switching flanks of the carrier pulses; and        iii) As a rule, the scanning signals are phase-shifted by 90° in relation to each other. The sine signal, as well as the cosine signal, must have the same storing time, otherwise an effective storing time of the position measuring system which differs from the exact position is obtained, which fluctuates between the storing time of the sine signal and the cosine signal. Regarding the position determination within a signal period, the respective scanning signal located in the vicinity of its crossover is decisive, since it shows the greatest change in position, or phase relation, in this range.        
A position measuring system is described in DE 44 10 955 A1, in which the light source is supplied with a strong current at the time an external trigger signal is present as a request signal. The disclosed synchronization of the light source with external trigger pulses is suited only for low demands made on the storing jitter, because the supply of the trigger pulses (request signal) to the light source located in the position measuring system takes place there by a wire connection from an external electronic tracking device (electronic control device). With customary cable lengths of 0.5 to 20 m, this does not assure a sufficient running time stability in the 10 ns range and below, and can therefore not be used in demanding applications. In this connection it should be noted that low-jitter trigger pulses are mostly available only directly at the electronic components which generate them.